Two-piece cans for beer and other pressurized beverages generally have a reentrant convex shape for their bottom. This permits the use of thinner metal while resisting distortion of the can by the pressure of the beverage.
Coatings are generally used on the inside of beverage cans to prevent corrosion and to protect the flavor of the beverage. Coatings are also used on the outside of beverage cans to facilitate decoration and identification of the can and its contents. However, if the can is made of adequately corrosion resistant material such as aluminum or tin-plated steel of an appropriate quality, it is generally not necessary to apply any coatings to the outside of the bottom of the can.
Although it probably is unlikely, some think it conceivable that a concave can bottom of uncoated shiney metal, especially bright aluminum, could act as a reflector to concentrate the sun's rays and possibly start a fire in dry grass or underbrush if a used can is discarded by the roadside or in the woods.
Exterior coatings are generally applied to two-piece cans after the can body has been formed. If they were applied before forming, they could be abraided or damaged by the can forming process. And it is difficult to spray a matte coating on the bottom of a formed can without overspraying onto the sides.